Laser sources, combined with rotating beam deflection mechanisms, are known for use in reading image information (also known as input scanning), exposing or printing image information (also known as output scanning), and display of image information. Prevalent use is made of holographic beam deflectors, which intercept a stationary beam of light, such as from a laser source, and which are rotated to cause the beam to scan.
Holographic beam deflectors typically comprise a carrier substrate in the form of a hologon disk that is mountable for rotation about an axis at the center of the disk. The disk may be transparent or opaque depending on whether the holographic beam deflector is of the transmission or reflection type. The disk may be regarded as divided into a plurality of sector-shaped facets, that is, regions bounded by two radial lines extending radially from the axis of the disk and an arc concentric with the disk, the arc usually being a portion of the circular periphery of the disk. The angles, included by the radial lines bounding each of the facets, are typically the same and sum to 360.degree.. Each facet includes a surface relief diffraction grating pattern usually formed in a photoresist layer carried by the carrier substrate disk, or a volumetric diffraction grating pattern in suitable material on or within the substrate. The lines of the diffraction grating pattern of each facet may be "radial", that is, parallel to the radius which bisects the facet, or they may be "tangential", that is, perpendicular to the radius bisecting the facet.
In a conventional multi-facet holographic beam deflector, a single diffraction grating pattern in each facet is provided to diffract a monochromatic (single wave length) light beam. For example, Kramer, in U.S. Pat. No. 4,289,371 discloses a plane linear grating disk for use as the beam deflection element in a holographic scanning apparatus. The scanning beam produced by such an apparatus is made wobble-insensitive if the incident and diffracted angles of the monochromatic beam illuminating the holographic disk are kept essentially equal. By use of linear, non-focusing gratings, such a hologon is also insensitive to errors in the centration of the disk on its rotational axis. The disclosed apparatus provides high resolution output with negligible banding without resort to non-spherical corrective optics.
A hologon-based beam scanner that is capable of providing a scanning beam having plural wavelength components, whereby the beam components would scan an image plane in a simultaneous and collinear fashion, would be quite advantageous for exposure of a multichromatic light sensitive medium, such as color photographic film. However, the conventional hologon-based scanner, such as the aforementioned proposed hologon scanner by Kramer, is unsuitable for the production of a multichromatic scanning beam because the conventional diffraction grating is dispersive. That is, the grating tends to split a multichromatic input beam into separately diverging component wavelength beams that do not scan in a collinear fashion.
The foregoing condition is based upon the known predisposition of a multichromatic beam, when diffracted by a linear plane diffraction grating having parallel grating lines, to separate into constituent monochromatic beams which exit the grating at different angles, as shown by the grating equation: EQU sin .theta..sub.i +sin .theta..sub.d =.lambda./d
where
.theta..sub.i and .theta..sub.d are incidence and diffraction angles, respectively, of the input beam; PA1 .lambda. is the wavelength of the input beam; and PA1 d is the grating pitch
For example, if .theta..sub.i is fixed at 45.degree., and .lambda. varies from red, to green, to blue, the angle .theta..sub.d will change radically. In a typical hologon scanner, a 3-color input beam would be diffracted into three scanning beams which expose three non-coincident scan lines.
Furthermore each scan line will have differing length and will exhibit a non-linearity (bow) which, when accumulated in the plural scan lines that fill an 81/2" page, would be equivalent to hundreds of pixels. Even if plural, single-wavelength beams (instead of a multichromatic input beam) were incident on the hologon disk at three different input angles so as to produce a composite, collinear output beam, the well-known desired minimum wobble condition could be satisfied for only one wavelength component in the composite output beam. Also, the scan line length and scan line bow would be different for each wavelength component in the output beam. The resulting scan line(s) would be unsatisfactory for most applications. These deleterious effects are known in the art and can be calculated using known beam trajectory equations; for example, as disclosed by R. A. Stark in U.S. Pat. No. 4,707,055.
The use of a single hologon for diffracting incident beams of differing wavelengths into at least three output beams has been proposed by Locke, in U.S. Pat. No. 3,795,768. Unfortunately, the resulting beam spots are not superimposed and the disclosed hologon is comprised of a thin reflective relief hologram, which cannot be made wobble insensitive.
One alternative approach has been proposed by McMahon et al., in U.S. Pat. No. 3,619,033 and in "Light Beam Deflection Using Holographic Scanning Techniques", in the February 1969, Vol. 8, No. 2, issue of APPLIED OPTICS. Disclosed is an apparatus for recording a three-color display, wherein the apparatus uses three separate color channel holograms (holographically-formed diffraction gratings) located at different radial positions on a photographic plate. Hence, the respective holograms are radially-separated and do not overlap. The apparatus is said to provide three output beams that are focussed to the same spatial position.
However, the fabrication of the proposed holograms in the apparatus disclosed by McMahon et al. will require an exposure system using illumination beams of three different colors. Also, the respective gratings must be separately located on the photographic plate or other substrate. A plural-wavelength exposure beam scheme is more difficult to accomplish than if a single wavelength exposure beam were used. Further, the plural-wavelength exposure beam scheme severely limits the materials that may be used in generating surface relief diffraction gratings, because the exposure material of choice (photoresist) is generally sensitive only to blue or ultraviolet light.
The separately-located gratings may occupy so much area that the resulting hologon disk or plate is undesireably large. The speed of rotation of the disk, and thus its scanning rate, is therefore limited because the aerodynamic drag and the centrifugal stress on the disk increase greatly as the hologon diameter is increased. A small hologon is preferable, as the drag on a disk when spun is proportional to approximately the fifth power of the disk diameter, and the stress induced in the disk due to centrifugal force is proportional to the second power of the disk diameter. The cost of diffraction grating fabrication also decreases as the disk diameter decreases.
The prior approaches to a multichromatic beam deflection apparatus, including those described by McMahon and Locke, also have a tendency to produce scan lines that are arcuate, which is undesireable in some applications. Further, the prior approaches typically incorporate either a lenticular (focusing) grating or a lenswheel--devices which are known to be sensitive to centration error and to wobble.
Kramer, in U.S. Pat. No. 4,848,863, discloses a hologon scanning apparatus which uses sequentially-arranged single wavelength gratings formed in respective facets in a disk, or in separate plates. Each grating is optimized to one wavelength of light in a multi-wavelength (red, blue, green) incident beam. Each facet or element has a different grating period, and all have the same .lambda. over D (wavelength to grating period) ratio. The facets or elements are moved serially to successively intercept the composite, multi-wavelength beam. The beam spots from each wavelength in the respective output beams are said to overlap and scan essentially collinear lines, successively, on the image surface.
However, because only a single wavelength scan line is provided from each facet or element, at least three facets or elements, for example, must be rotated through the incident beam to provide three respective collinear scan lines. The available scan rate of the proposed apparatus is therefore less than desired. Further, because the disclosed gratings are separately spaced (so as to be sequentially illuminated), the hologon disclosed by Kramer is necessarily larger and more cumbersome than is desired.